Date of Award
Doctor of Philosophy (PhD)
The maturation of antibody affinity during the immune response, discovered as a serological phenomenon in 1964, is critical to the development of high affinity humoral immunity. This process takes place in germinal centers (GCs), which are microanatomical structures composed of antigen-specific B lymphocytes that form in secondary lymphoid organs upon infection or immunization. High affinity B cells are selectively expanded in the GC by iterative rounds of migration between a zone of hypermutation and proliferation (the dark zone, or DZ) and a zone of selection (the light zone, or LZ). The mechanism whereby somatic antibody mutants are selected on the basis of affinity has been elusive. In the first part of this thesis, I demonstrate that affinity-based selection in the GC operates through regulation of proliferation and hypermutation. B cells with affinity-enhancing mutations capture more antigen through their BCRs for presentation as peptide-MHCII to CD4+ T cells. In turn, enhanced T cell help induces selected B cells to return to the DZ, where they spend a prolonged period of time proliferating in between rounds of competition in the LZ. By dividing a greater number of times during each passage through the DZ, high affinity cells can expand within the population and are provided more opportunities to hypermutate their antibody genes. In the second part of this thesis, I examine the regulation of cell cycles in the DZ during GC selection. Transcriptional profiling revealed that T cell help enhanced signaling through the E2F and c-Myc pathways within selected GC B cells in the DZ. Moreover, T cell help increased the speed with which selected GC B cells progressed through both the S and G2/M phases. Genome sequencing and single-molecule analyses revealed that S phase is regulated by increasing the speed of DNA replication forks, while maintaining the dynamics of replication origin activation throughout the genome. Thus, T cells mediate affinity-based selection in the GC by inducing selected cells to reside longer in the DZ where they undergo accelerated cell cycles.
Gitlin, Alexander, "Selection Dynamics in the Germinal Center During Antibody Affinity Maturation" (2016). Student Theses and Dissertations. 303.